This invention relates to converter systems for converting AC power into DC power with suppression of higher harmonics at the installation point.
In the case where semiconductor converter circuits, such as inverters, bridge thyrister converters, thyrister motors utilizing cyclo-converters, etc., are coupled directly to power supply systems, higher harmonics generated by the converter circuits flow into the power system which usually have smaller impedance, thereby generating a harmonic distortion in the source power supply.
Thus, voltages having harmonic distortions are supplied to the loads connected to the power supply system. These higher harmonics may overheat the electrical components, especially capacitors supplied therewith, sometimes giving rise to insulation failures. When, on the other hand, reactors are coupled in series with the capacitors for preventing the higher harmonics from flowing into the capacitors, abnormal noise or temperature rise may be observed in such reactors.
For the purpose of reducing the damage caused by the higher harmonics, passive filters consisting of passive elements such as reactors and capacitors, or active filters consisting of active elements, are utilized. These filters, however, have some resistance, which increases insertion energy loss and thus reduces the efficiency of the system.
FIG. 1 shows a conventional system for suppressing higher harmonics. A branch line 3 is connected to an intermediate node point 4 between the power supply side 1 and a load side 2, of the higher harmonics suppressing system. The outputs of the instrument transformers 40, 41, and 42, which detect the voltage at the branch point, the load current at the side of the user load, and the current at the power system side, respectively, are introduced to a measurement/monitor/control circuit 5, which measures and monitors the higher harmonics. A main filter circuit 6, controlled by the measurement/monitor/control circuit 5, is connected to the node 4 for absorbing the higher harmonics.
In the system of FIG. 1, the higher harmonics being generated from electrical components connected to the user load side 2 and flowing into the power supply side 1 are branched into the filter circuit 6 to be absorbed therein. Thus, the higher harmonic contents flowing into the power supply side 1 are reduced.
This system for suppressing the higher harmonics is good, both in principle and in practical operation. However, the cost of installation may become substantial if such systems are to be provided for each one of the loads connected to the power supply system.
Among the loads coupled to the power source, there are those which require fine waveform control that is effected by means of inverters, cyclo-converters, or the like. However, there are also loads with large inertia, such as electrical resistance heaters or electrolytic cell loads, for which small fluctuations do not matter and only the time integral of the power or that of current supplied thereto is significant for their operation. In the case of these loads, small fluctuations in the waveform have little effect on their operation. That is, if the loads are responsive only to the amount of heat or electric quantity which corresponds to the time integral of the power or current, the instantaneous fluctuations in the waveforms are filtered and smoothed due to the integrating function of the load and, hence, have no vital effect on the whole.